1. Field of the Invention
The invention relates to a drilling tool according and more particularly for drilling in hard substances such as rock or concrete with axially striking drive machines.
2. Discussion of the Background
EP-B1-0,126,409 has disclosed a drilling tool of the type as defined in the preamble. In this known drilling tool, the drilling-dust flute does not run at constant inclination over its entire length. On the contrary, it has its smallest inclination in the area of the drill head, while it preferably has steadily increasing inclination in the remaining area. In this way, the uniform spacings, present in drilling tools having uniform inclination, between the abrupt changes in cross-section which result at the transition of the drilling-dust flute to the helix webs are avoided. According to the findings in this prepublication, uniformly spaced abrupt changes in cross-section result in disadvantages with regard to the superimposition of vibrations, resonance phenomena and sound emission. To optimally convert the striking energy of the striking impulse and thus to convert a higher cutting energy as well as to obtain a decrease in the sound emission, the known device therefore proposes a steadily changing inclination in the conveying helix. In particular exemplary embodiments, sectional areas are also provided with uniform inclination, but they extend over a different number of leads. However, the essential point of the known device is to obtain, in the area of the drill head, a conveying helix having small inclination and thus a large lateral supporting surface as a drill guide surface. However, this inclination is to increase steadily or continuously in the direction of the drill chucking shank.
Furthermore, in the known publication, it is stated in columns 5 to 7 that the drilling-dust flutes in the area of small angles of inclination, i.e. in particular in the area of the drill head, have a virtually rectangular cross-section, the respective back face or outer circumferential face of the flute web forming virtually a 90.degree. flank angle to the drilling-dust carrying surface. Accordingly, in the conveying-helix area having small inclination, relatively wide drilling-dust carrying surfaces for the accumulating drilling dust and thus reliable disposal flutes for the drilling dust are formed.
Furthermore, the known publication points out that the rectangular cross-sectional shape of the drilling-dust flutes is not necessary in the area of large flute inclination. As the inclination of the conveying helix increases in the direction of the chucking end of the drill, the drilling-dust carrying surface adjoining the lateral back face of the flute webs becomes narrower and narrower, the at first 90.degree. flank angle continually increases and finally forms merely a curved connection to the respective back face. Adequate feed and transport of the drilling dust is guaranteed by the large flute cross-section (at increasing inclination) (at increasing inclination) flute 90.degree., the drilling-dust flute.
In this respect, it is also pointed out in the known publication that, even in conveying-helix areas having a large inclination, the approximately rectangular cross-sectional shape of the disposal flutes could of course be retained, i.e. a rectangular cross-section over the entire length of the drilling-dust flute.
This known publication therefore shows drillingdust flutes with varying inclination, the carrying surface of the drilling-dust flute in areas of small inclination being designed as a wide carrying surface virtually at right angles to the lateral back face of the flute webs, while, as inclination increases, this carrying surface runs as a carrying surface which becomes narrower and has a gradually curved transition to the lateral back face of the flute webs. The surface normal, i.e. a perpendicular force vector on this drilling-dust carrying surface, therefore also changes in its direction when inclination changes.
Drilling-dust flutes of this type are described in German Patent Specifications No. 1,291,707 and 1,927,754. Here, the drilling-dust carrying surfaces are shown as undercut, bulbous or pocket-shaped rounded portions whose radially outer transition surface to the lateral circumferential face of the drill or back face encloses an acute flank angle which is likewise approximately 90.degree. , and preferably 75.degree. to 80.degree. (see Patent Specification 1,927,754, column 4, line 37 ff.) If this specified flank angle .beta. is&lt;90.degree., the undercut drilling-dust flute is designated by a "positive" flank angle. If the transition of the carrying surface to the side surface forms an angle .beta.&gt;90.degree., this is designated by a "negative" flank angle.
The change in the drilling-dust carrying surface arising in EP-No. 0,126,409 mentioned above, as a result of the changes in inclination therefore corresponds to a gradual transition of the neutral flank angle (.beta..apprxeq.90.degree.) to a negative flank angle (.beta.&gt;90.degree.).